The present invention relates to an oxidation process particularly for alcohols and especially to the oxidation of a secondary alcohol to a ketone.
Various ketones are employed as chemical intermediates, for example in the synthesis of pharmaceuticals, one of which intermediates is 1,3-dichloroacetone. Currently, the ketone is produced by oxidation of the corresponding alcohol or chlorination of acetone, but both routes are subject to substantial practical problems. In the former route, there are difficulties of contamination of the product with the oxidant and the purification or other disposal of the effluent and in the latter route there is a severe risk of overchlorination, which inevitably reduces the purity and yield of the product.
In principal, there are many strong oxidants that might be expected to oxidise alcohols but they may also otherwise react with them and/or other substituents of the alcohol-containing molecule. One such oxidant comprises bromine. A paper by N. Venkatasubramanian and V. Thiagarajan in Tetrahedron Letters No. 14 pp 1711-1714 published by the Pergamon Press in London 1968 reviewed various possible mechanisms proposed for the bomine/alcohol reaction both by themselves and earlier by Deno and Potter in JACS 82, 406 (1967), Swain, Wiles and Bader in JACS 83, 1945 (1961) and Barker, Overenk and Rees in JCS 3263 (1964). The disclosure indicates that the presence of a strong electronegative substituent adjacent to the hydroxyl group retards the oxidation reaction considerably. On the other hand, it has also been recognised that ketones in particular are susceptible to bromine substitution reactions probably effected chemically by HBr/Br.sup.-. The studies leading to the present invention confirm that the reaction products can include both ketone and brominated compounds.
Bromine itself as a reagent is particularly unpleasant to handle, especially on an industrial scale because it is a toxic low boiling point liquid that can react with water to form toxic and corrosive fumes. Moreover, the resultant hydrogen bromide reaction product, in theory twice the molar amount of bromine introduced, poses considerable and therefore expensive waste disposal problems. It would, at least in theory, be attractive to employ a technique of regenerating bromine from hydrogen bromide, because such a procedure would reduce the problems of handling bromine and disposing of hydrogen bromide, but it follows that such a technique would inevitably maximise the exposure of the alcohol/ketone to contact with HBr/Br.sup.- throughout the entire reaction period and accordingly increase the extent of the unwanted bromine substitution reactions.